Abstract
Simple SummaryThe brown planthopper is a major rice pest in Asia that causes extensive damage where farmers apply large amounts of nitrogenous fertilizers. Over the last 50 years, host-plant resistance, i.e., a plant’s ability to deter planthoppers or reduce their survival and reproduction, has been the main focus of public research into planthopper management. However, there have been calls for increased attention to issues of rice tolerance against the planthopper, i.e., a plant’s ability to withstand attack and compensate for damage. We examined the interactions between planthoppers and 16 varieties of rice under varying levels of soil nitrogen. The varieties included susceptible, resistant and tolerant lines. We found that nitrogen generally increased planthopper fitness across the varieties; however, relative resistance was maintained in varieties with major resistance genes. Functional plant loss was found to be greatest in susceptible varieties after planthopper attack, and generally declined in resistant and tolerant varieties under increasing nitrogen levels. This increase in tolerance was most apparent among resistant and moderately resistant, large-sized, fast-growing varieties that were capable of overcoming relatively high weight and growth rate reductions.The brown planthopper, Nilaparvata lugens (Stål), is a key challenge to rice production in Asia. Outbreaks of planthoppers are associated with excessive fertilizer applications; consequently, we examined planthopper interactions with susceptible, tolerant and resistant varieties of rice under varying levels of soil nitrogen in a greenhouse experiment. We compared planthopper fitness (survival × reproduction) and plant tolerance (functional plant loss index) for 16 varieties at 0, 80 and 150 Kg added nitrogen ha−1. The planthoppers grew larger, developed more quickly and laid more eggs on susceptible varieties, compared with the resistant and tolerant varieties. Moreover, soil nitrogen generally increased planthopper fitness on resistant varieties, but relative resistance was maintained. Functional plant loss was highest among the susceptible varieties, but weight and growth rate reductions per mg of planthopper were often highest in the tolerant varieties. Tolerance was associated with large, fast-growing plants, with at least moderate resistance to the planthopper. Susceptibility was associated with a small size and/or an absence of resistance genes. Our results suggested that early-tillering rice plants can be both resistant and tolerant to the brown planthopper, but cannot be both susceptible and tolerant of planthoppers at high densities. This indicates that at least moderate resistance is required for tolerance against this herbivore. Furthermore, although dwarf varieties had a low tolerance of planthoppers, they could express resistance through functioning resistance genes.
Highlights
Resistance can be described as a plant’s capacity to repel attack and reduce damage from herbivores [1,2,3]
We examined the honeydew production from the honeydew test; the final nymph biomass; the proportion of nymphs at the adult stage; the proportion of adults that were female; the proportions of adult females and males that were brachypterous from the Survival test, and the number of eggs laid per plant from the Oviposition test using univariate general linear models (GLM)
Under conditions of high planthopper densities, plants were both resistant and tolerant to the planthopper, but could not be both susceptible and tolerant, indicating that at least moderate resistance was required for tolerance to the brown planthopper
Summary
Resistance can be described as a plant’s capacity to repel attack and reduce damage from herbivores [1,2,3]. Over the geographical range of a plant species, individual plants vary in their expressed resistance against specialized insect herbivores [4,5]. Resistance varies between different plant tissues and over the course of plant growth and development [6,7]. Plants that share the same nuclear genome (e.g., clones and pure-line crop varieties) may express different levels of resistance against herbivores depending on the plant’s environmental and growth conditions, as this relates to resource availability (e.g., light, space, water and nutrients) [8,9,10], or the effects of other biotic stresses (i.e., infestation by other herbivores or diseases) [11,12,13,14,15]. There are many well-documented cases of high fertilizer levels increasing plant susceptibility to herbivores
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